Gingrich, a former House speaker whose presidential campaign is on the rise just weeks before the first nominating contests take place, offered sharp criticism of Romney. For months, the Georgia Republican has refused to criticize his rivals and instead has kept his focus on President Barack Obama.

That all seems to be over. Branding the former Massachusetts governor as a political opportunist, Gingrich said it is one thing to change positions if new facts become available and quite another to shift positions for political gain.

"It's wrong to go around and adopt radically different positions based on your need of any one election, then people will have to ask themselves, 'What will you tell me next time?'" Gingrich told WSC-FM radio Monday morning ahead of a three-day campaign swing through South Carolina.

Romney has changed his positions on gay rights and abortion since his first political campaign in 1994. Since that unsuccessful effort, he has publicly shifted rightward.

It isn't enough to convince some, including Gingrich.

If the player does not load, please check that you are running the latest version of Adobe Flash Player.

"We think there has to be a solid conservative alternative to Mitt Romney," Gingrich said.

At times Gingrich has blamed the media for stoking the divisions among the contenders in an attempt "to get Republicans fighting with each other," as he put it during an NBC debate. "You want to puff this up into some giant thing," Gingrich said then.

Gingrich has seen his political standing rise as he has posted solid debate performances and laid the groundwork for a traditional campaign. In South Carolina, for instance, he has five offices and his supporters are making thousands of phone calls every day.

Gingrich packed a town hall-style event Monday night at the College of Charleston. Supporters who couldn't get in lined up around the block in the hopes that organizers would let more people inside and out of the pouring rain.

While Gingrich's two divorces and admissions of infidelity are unlikely to endear him to Christian conservatives who have a great sway here, he is pitching himself as the candidate who can best challenge Obama, who is deeply unpopular among Republicans.

"No American president has the right to side with foreigners against the people and laws of the United States," Gingrich told the College of Charleston audience, questioning Obama's allegiance.

But Gingrich knows his own shortcomings.

"No person except Christ has ever been perfect," Gingrich told WSC-FM. "So I don't claim to be the perfect candidate. I just claim to be a lot more conservative than Mitt Romney and a lot more electable than anybody else."

He added: "I'm the one candidate who can bring together national security conservatives and economic conservatives and social conservatives in order to make sure we have a conservative nominee."

Gingrich also faces criticism for how he spent his time after stepping down as the top Republican in the House. He built a network of advocacy organizations, think tanks and consulting firms. He insists he has never lobbied and touted his private sector experience: "I think we do very good work."

And Gingrich, too, is facing questions about his position on immigration. His rivals have suggested it would offer amnesty for illegal immigrants.

"I'm for controlling the border. I am against amnesty. I'm very disappointed that at least one of my friends has been, for the last four days, going around saying things that she knows are not true," Gingrich said, taking on Michele Bachmann, the Minnesota congresswoman who has seized on the issue.

Gingrich has proposed that local communities have the power to determine whether their neighbors can remain in the United States despite their immigration status.

"We ought to have a citizen certification board in every community and citizens should make the decision whether that person should get a path to legality but not citizenship — no right to vote, doesn't become a citizen," he said.